Centrifugal cleaner

ABSTRACT

Apparatus for separating solid particles from liquid in a slurry in which radial blades are attached to a rotating base plate. The blades slope in a direction opposite the direction of rotation whereby the trailing edge of the blade is spaced from the base plate. Means are provided for delivering the slurry onto the inner end of the impellers near the surface of the base plate and for rotating the disc. The solid particles move rapidly in a radial direction along the impeller blades. That rapid movement creates a Coreolis force that causes the particles to be discharged from the trailing edge of the blades. Because the liquid wets the blades and the viscosity of the liquid slows movement of the liquid on the blades, Coreolis forces on the liquid are relatively small and the liquid is discharged from the outer end of the blades. The solid particles and liquid are collected separately as they are discharged from the impeller blades.

United States Patent [191 Gaylord 1 Feb. 5, 1974 CENTRIFUGAL CLEANER [75] Inventor: Eber W. Gaylord, Pittsburgh, Pa.

[73] Assignee: Gulf Research & Development Company, Pittsburgh, Pa.

22 Filed: Aug.11, 1971 21 Appl.No.:170,839

[52} US. Cl 209/505, 209/18, 209/199, 210/512 [51] Int. Cl B01d 21/26 [58] Field of Search..... 210/84, 273, 377, 520, 378, 210/380; 209/198, 199, 211, 465, 505, 18

Primary Examiner-Samih N. Zaharna Assistant Examiner-F. F. Calvetti 5 7] ABSTRACT Apparatus for separating solid particles from liquid in a slurry in which radial blades are attached to a rotating base plate. The blades slope in a direction opposite the direction of rotation whereby the trailing edge of the blade is spaced from the base plate. Means are provided for delivering the slurry onto the inner end of the impellers near the surface of the base plate and for rotating the disc. The solid particles move rapidly in a radial direction along the impellerblades. That rapid movement creates a Coreolis force that causes the particles to be discharged from the trailing edge of the blades. Because the liquid wets the blades and the viscosity of the liquid slows movement of the liquid on the blades, Coreolis forces on the liquid are relatively small and the liquid is discharged from the outer end of the blades. The solid particles and liquid are collected separately as they are discharged from the impeller blades.

15 Claims, 6 Drawing Figures PAIENTEB FEB 51914 SHEH 3 0F 4 1 CENTRIFUGAL CLEANER This invention relates to the cleaning of solid particles and more particularly to the separation of solid particles from a liquid in which the particles are suspended.

Frequently solid particles are suspended in a liquid during one stage of an operation and it becomes desirable later to separate the solid particles from the liquid for reclaiming the solid particles. For example, in some ore dressing an ore may be suspended in water during several steps of a process and then separated from the water for further treatment. Another process in which such separation is desirable is the manufacture of steel shot in which finely divided droplets of steel are quenched in water and the shot particles must be separated from the water for grading according to size and shape.

Recently, a process known as abrasive jet drilling has been developed for drilling wells through hard formations. In that process, a slurry of abrasive particles, preferably steel shot having a size in the range of 20 80 mesh, is discharged at a high velocity against the bottom of the borehole to erode the borehole. The resultant mixture of drilling mud, abrasive particles, and rock cuttings is circulated up the well to the surface where the drilling mud is reconditioned for further use. It is desirable to separate and discard from the drilling mud broken abrasive and both large and small rock cuttings before the drilling mud is recirculated through the well. Because steel shot particles will not flow readily when damp or will settle out of drilling mud unless the drilling mud is moving rapidly, handling of the drilling mud at the surface during the reconditioning is difficult. It has been proposed to dry the steel shot to facilitate screening it and introducing it into the drilling mud at a uniform rate for further drilling operations. When the shot is wet with some type of drilling mud used in the abrasive jet process, it is essential to prevent clumping of the shot to separate more drilling mud from the shot than is possible with a magnetic separator or screens before delivering the shot to a drier for evaporation of moisture from the surfaces of the shot. Regardless of the type of drilling mud used, it is highly desirable to separate the shot and drilling mud liquids as completely as possible to reduce the cost of drying the shot.

The usual equipment for separating solids and liquids is not satisfactory for removal of shot particles from drilling mud used in the abrasive jet drilling process. Screens leave an objectionably large amount of drilling mud on the shot. Screens cannot handle abrasive laden drilling mud at a high rate; therefore, very large and excessively expensive screens would be required to handle shot-laden drilling mud at the rates at which drilling mud is circulated in the abrasive jet drilling process. Moreover, the highly abrasive nature of steel shot has caused wear of the screens at a very rapid rate precluding their use in a continuous process.

Centrifuges are frequently used to separate solids from liquids, but centrifuges are expensive, have a relatively low throughput and a short life when handling abrasive particles. Cyclone spearators are effective only in reducing the moisture content on the shot discharged from the separator to the range of about percent by weight. The cost'of evaporatingwater from shot particles discharged from a cyclone separator to produce shot dry enough to be readily handled is excessive. Another disadvantage of cyclone separators is that pumps of high capacity are required to deliver the slurry into the cyclones at rates high enough to cause effective separation and the abrasive causes rapid wear of such pumps.

This invention resides in apparatus for separating solid particles from a liquid which includes a rotating impeller having a base plate on which are mounted a plurality of impeller blades inclined with the trailing edge spaced from the base plate. A guide directs the mixture of liquid and solid particles onto the impeller I blades near the base plate and at the end of the impeller. Separate collection chambers are provided for the liquid and solid products discharged from the impeller blades. The low resistance to movement of shot particles over the impeller blades causes the shot to move rapidly in a radial direction and gives the shot particles Coreolis force whereby the shot particles are discharged from the impeller blade at its trailing edge. The wetting of the surface of the blade by the liquid and the viscosity of the liquid cause the liquid to move slowly in a radially outward direction and thereby reduce the Coreolis force on the liquid to such an extent that the centrifugal force on the liquid predominates to such an extent that the liquid is discharged from the outer end of the impeller blades.

In the drawings:

FIG. 1 is a diagrammatic view, principally in vertical section, of a preferred embodiment of the solid particle cleaner of this invention.

FIG. 2 is a vertical sectional view taken along the section line Il-ll in FIG. 1 of a preferred embodiment of the impeller blade and a portion of the base plate of the impeller.

FIG. 3 is a plan view of the base plate of the impeller.

FIG. 4 is a bottom view of the impeller illustrated in FIG. 1.

FIG. 5 is a vertical sectional view in a plane parallel to the axis of rotation of the base plate, and looking toward that axis, of a second embodiment of an impeller blade that can be used in this invention.

FIG. 6 is a fragmentary vertical sectional view of apparatus employing this invention in which the impeller blades are mounted on the upper surface of the base plate.

For convenience in the description of this invention, the apparatus will be described for cleaning steel shot used in abrasive jet drilling. The apparatus is not limited to that use but can be used advantageously in separating solid particles from a liquid. Because the separation depends on the difference in resistance to movement along the impeller blade, the apparatus also provides separation of solid particles of one shape from solid particles of a different, principally less-rounded shape, and, for that reason, can be useful in separating broken shot from the shot that is to be reused.

Steel shot used in the abrasive jet drilling preferably has a size in the range of 20 mesh, and still more desirably in the range of 30 50 mesh in the U. S. Sieve Series. It is suspended in a drilling mud in a concentration of 2 percent 20 percent, and preferably 2 10 percent by volume. A preferred drilling mud for the abrasive jet drilling process contains relatively small amounts, of the order of l 4 percent by weight, of clay solids dispersed in water and about 2 5 percent by weight of finely divided cellulose fibers. The drilling mud will, additionally, contain some finely divided rock cuttings. In an improved process for the treatment of such drilling mud to recondition the mud discharged from the well for reuse, the drilling mud discharged from the borehole of the well is passed over a shale shaker to remove large cuttings and is then passed through a magnetic separator which separates from the main portion of the liquid of the drilling mud a slurry containing approximately 50 percent by bulk volume of steel shot. The term 50 percent bulk volume is used to indicate a slurry in which the upper level of the solid particles, if they were allowed to settle in a 100 cc. graduate cylinder from 100 cc. of the slurry, would be at the 50 cc. mark. The 50 percent slurry is charged to the solid particle separator of this invention.

Referring to FIG. 1, a shot cleaner indicated generally by reference numeral includes a housing 12 of circular shape in horizontal cross section having a cover 14 closing its upper end. The lower end 15 of housing 12 slopes downwardly to an outlet 16 for discharge of the drilling mud, as is hereinafter described. Within the housing 12 is a shot chamber 18, also of circular shape in horizontal cross section and of a diameter smaller than the housing 12 to provide an annular space 20 between the wall of the shot chamber 18 and the housing for collection of the drilling mud separated from the shot. Supported directly above the shot cleaner 10 by a beam 22 is a motor 24 having a shaft 26 extending vertically downwardly through an opening in cover 14. A hub 28 on the lower end of shaft 26 provides support for an impeller, indicated generally by reference numeral 30.

Impeller 30 includes a base plate 32 secured to hub 28 by suitable studs 29. As is best shown in FIG. 3 of the drawings, base plate 32 has a plurality of uniformly spaced radial slots 34 extending outwardly from an inner end 35 which may be, for example, at the approximate location of the periphery of hub 28. Slots 34 can extend for any convenient distance toward the periphery of base plate 32; and in the embodiment of the invention illustrated in the drawings, the slots extend for approximately one-third the distance from the outer edge of hub 28 to the outer edge of the base plate 32. Secured to the lower surface of the base plate 32 by means of braces 36 bolted to the base plate are a plurality of impeller blades 38. Impeller blades 38 illustrated in FIG. 1 have a tab 40 that extends through the slot 34.

It is essential to this invention that impeller blades 38 are held by braces 36 in a sloping position, as is best shown in FIG. 2 of the drawings, with the leading edge 42 adjacent the base plate 32 and the trailing edge 44 spaced from the base plate. In the embodiment of the invention illustrated in FIGS. 1 and 2, the impeller blades 38 are curved at their trailing edge through an arc of approximately 90 in a direction opposite the direction of rotation of the impeller. A single horizontal slot 46 is shown in FIGS. 1 and 2 extending through the blade at the juncture of the curved portion of the blade with the flat leading portion thereof. In some instances it may be desirable to have additional horizontal slots between slot 46 and the base plate. The outer ends 48 of the blades 38 are located approximately directly above the wall of the shot chamber 18.

Welded to the inner edges of the blades 28 so that it will rotate integrally therewith is a guide 50. Guide 50 is in the form of an inverted cone with an upwardly opening base spaced below the base plate 32 whereby ports 52 are defined by the lower surface of the base plate, the upper surface of the guide 50, and the impeller blades 28. With the arrangement shown in FIG. 1 with the tabs 40 extending through the slots 34 adjacent the ports 52, a continuous surface of the impeller blade extends completely across the slots 52 to allow uninterrupted movement of the shot and mud onto the impeller blade. The lower end of the guide 50 has an inlet sleeve 53 to receive a feed line 54.

In the operation of the shot cleaner illustrated in FIG. 1, the motor is started and rotates the impeller 30 at a speed that will impart the necessary centrifugal force and Coreolis force to make the desired separation. Rotation at approximately 1,750 rpm has been found to be adequate for separation of steel shot from drilling mud. The mixture of drilling mud and shot particles is pumped through feed line 54 and discharged into the guide 50. Centrifugal force generated by the rotation of the impeller causes the mixture to flow upwardly along the conical surfaces of the guide 50 into ports 52 and then onto the surface of the impeller blade. The shot particles move freely and rapidly over the surface of the impeller blades toward the outer end 48 of the impeller blades. Because of the speed of the movement of the shot particles in a radial direction, they do not have time to acquire the increased linear velocity of the impeller at greater distances from the center of rotation; hence the shot particles have a high Coreolis force toward the trailing edge 44 of the impeller blade and are discharged from the trailing edge into the shot chamber 18. A typical path of the shot particles is indicated in FIG. 1 by dashed line 56.

The drilling mud in the mixture charged to the shot cleaner wets the surface of the impeller blade. Because of the wetting of the surface and the viscosity of the drilling mud, the radial flow of the drilling mud is substantially slower than of the shot and as a result the drilling mud has time to acquire a speed in the direction of rotation more closely approaching the speed of the impeller blade. The centrifugal force on the drilling mud causes it to continue the radially outward flow until it is discharged from the outer end 48 of the impeller blade. Typically, the drilling mud will follow a path between the lines 58. The drilling mud is, therefore, discharged into the annular space 20 between the housing 12 and the wall of shot chamber 18 and flows downwardly to the outlet 16.

The slots 46 in the impeller blade 38 and the curved trailing edge 44 alow an increase in the rate at which the mixture of drilling mud and shot particles can be put through the shot cleaner and still obtain the desired separation. An increase in rate of delivery of drilling mud to the impeller causes the drilling mud to spread over a larger area of the blade as it flows toward the outer end of the blade; therefore more of the drilling mud flows toward the trailing edge 44 of the blades.

Drilling mud reaching the slots 46 and flowing through them continues its movement in the radial direction and is struck by the following impeller blade at a larger distance from the center of rotation. The function of the slots is, therefore, to interrupt movement of the liquid toward the trailing edge of the impeller blade while movement in a radial direction continues. Liquid that passes over slot 46 moves along the curved surface between the slot 46 and the trailing edge 44. That liquid follows the curved surface and is discharged from the trailing edge 44 at a level above the lowest level of '5 the impeller blade. That liquid, too, is struck by the leading surface of the following impeller blade at substantially the same level as the trailing edge 44 but at a greater distance from the center of rotation, and, therefore, has additional time to reach the outer end of the impeller blades.

The shot particles have sufficient momentum to pass directly over the slot 46 and do not follow the curved surface of the impeller blades as the liquid does. The shot leaves the impeller below the trailing edge of the impeller blades rather than from the outer end of the blades. In the apparatus illustrated in FIG. 1, the shot chamber is open at its lower end whereby the shot particles can fall directly onto a suitable conveyor indicated by reference numeral 60 for transporting the shot to equipment for further processing.

Reduction of the amount of water adhering to the surface of the shot to approximately 1 percent by weight of the shot has been obtained with the apparatus illustrated in FIG. 1. The degree of separation is influenced by the design of the impellers, their inclination with the base plate, and the rate of rotation of the impeller. Best separation of steel shot has been obtained if the angle between the base plate and the impeller blade is in the range of about 35-50. An impeller rotation rate of 1,750 1,800 rpm is effective in separating steel shot from drilling mud.

The degree of separation of the shot from the drilling mud can also be controlled by the location of the walls separating the drilling mud taken from the impeller blade from the shot. The drilling mud covers a band on the impeller blade with the ratio of drilling mud to shot increasing in the direction of the outer end of the impeller blade. For convenience in adjusting the separation, a shroud 62 can be secured to the upper end of the shot chamber 18. Control of the amount of moisture on the shot can be obtained by control of the location of the upper edge of the shroud. If it is desired to have a very low moisture content in the separated shot, a shroud having its upper edge near the center of rotation of the impeller is secured to chamber 18. If it is desired to discharge less shot with the drilling mud, a shroud having its upper edge at a greater distance from the center of rotation is secured to chamber 18.

It is not essential to this invention that the impeller blade extend downwardly from the base plate or that the shot particles be made to discharge from the lower edge of the impeller blade. The dynamic forces causing the separation are the result of rotation of the impeller blade and are large enough, compared to the force of gravity, to cause the shot particles to move over the surface of the blades toward the trailing edges regardless of the orientation of the impeller base plate. In the apparatus illustrated in FIG. 6, the base plate 64 of the impeller is secured to the upper surface of a hub 66 mounted on a rotating shaft 68 driven by a motor, not shown. Impeller blades 70 are welded to brackets 72 secured to the impeller base plate 64 to support the impeller blades above the impeller base plate with their upper edge 74 trailing the lower edge. The impeller base plate also could be oriented in any position between one in which the impeller blades extend downwardly from the base plate as in FIG. 1 and extend upwardly as in FIG. 6. The impeller blades 70 illustrated in FIG. 6 do not have either the curved trailing edge or slot of the blades 38 but are merely flat, rectangular sheets inclined to the base plate at an angle of 35- 50,

but could be identical to the impeller blades illustrated in FIG. 1.

A guide ring 76 is mounted on the upper surface of the base plate 64 in a manner such that the guide ring rotates integrally with the impeller. Guide ring 76 can be welded to the inner edge of the impeller blades or welded directly to the base plate. In the embodiment illustrated in FIG. 6, guide ring 76 has a plurality of openings 78 positioned a short distance above the base plate 64 for the discharge of the mixture of solid particles and liquid onto the surface of each of the impellers. By locating the slots a short distance above the base plate 64, a smooth unbroken flow onto the impeller blades 70 can be obtained without extending the blades through the base plate. An inlet line 79 opens downwardly into the guide ring to deliver the mixture of solid particles and liquid into the guide ring.

The operation of the cleaner illustrated in FIG. 6 is similar to that illustrated in FIG. 1 with the exception that the shot particles follow a path, indicated by dashed line 80, upwardly toward trailing edge 74 of the impeller blades 70. The shot particles moving in an upward and outward direction are discharged from the trailing edge of the blade and delivered outwardly over a shroud 82. The drilling mud wets the surface of the impeller blades 70 and flows radially outward at a lower speed, hence, is subjected to less Coreolis force. The drilling mud is discharged from the outer end 84 of the impeller blade against the inner surface of shroud 82 and is collected separately from the shot particles.

The arrangement of the impeller illustrated in FIG. 1 is preferred for the separation of steel shot particles from drilling mud because damp shot particles stick and tend to buildup on walls even though the walls may be vertical. In the embodiment illustrated in FIG. 1, the flow of the shot leaving the impeller blade is downwardly toward the open lower end of the shot chamber and buildup of shot particles on the wall of the shot chamber is minimized.

Another embodiment of impeller blade that can be used is illustrated in FIG. 5. Referring to FIG. 5, an impeller blade indicated generally by reference numeral 86 is secured to the upper surface of a base plate 88 by means ofa bracket 90. The impeller blade has a tab 92 at its leading edge extending through a slot in the base plate. The impeller blade 86 slopes from its leading edge 94 upwardly toward its trailing edge 96 in a direction opposite the direction of rotation of the impeller. Impeller blade 86 has a leading flight 98 that extends from the base plate upwardly and a trailing flight 100 secured to the upper end of the leading flight. The trailing flight slopes at an angle to the base plate less than, but not more than approximately 10 less than, the leading flight to provide a change in direction which aids the shot particles in leaving the surface of the impeller blade. The drilling mud follows the surface of the trailing flight of the impeller blade and stays on the impeller blade a longer time before discharge from the impeller blade and thereby increases the amount of drilling mud that reaches the outer end of the impeller blade. The impeller blade 86 has its leading surface covered with a coating material 102 to reduce the erosion of the impeller blade by the shot particles.

The separator of this invention utilizes the difference in resistance to flow across the surface of the impeller blades. Differences in density and particle size have an effect on the separation only extent that they influence mobility relative to the blades. While it is designed primarily for separation of solid particles from liquids, it is also advantageous in making'a separation between particles of one shape and particles of another shape if those two shapes cause differences in the resistance to flow across the impeller blades. For example, in the separation of steel shot from drilling mud that has been circulated at least once through the borehole of the well, some separation between unbroken shot particles and broken shot particles is gained. The broken shot particles have a lower degree of roundness than the shot particles and do not move as readily over the surface of the impeller blades. The broken shot particles are, therefore, carried on the blade a longer time and discharged from the impeller blade at a greater distance from the center of rotation of the impeller blades with the result that many of the broken shot particles are collected with the drilling mud.

I claim:

1. Apparatus for separating solid particles from a slurry comprising an impeller base plate, means for rotating the impeller base plate about a central axis perpendicular thereto, a plurality of uniformly spaced sub-- stantially radially extending impeller blades secured to the impeller base plate in an inclined position with the trailing edge of the blades spaced from the impeller base plate and the leading edge at the base plate, means for delivering the slurry onto the end of the impeller blades nearest the axis of rotation and adjacent the impeller base plate, said blades being constructed and arranged so that particles that move more freely on the blades are discharged from the trailing edge of the blades and material that moves less freely is discharged from the outer end of the blades and means for separately collecting material discharged from the trailing edge of the impeller blade and material discharged from the outer end of the blades.

2. Apparatus as set forth in claim 1 in which the impeller blades are curved along the trailing edge in a direction opposite the direction of rotation of the impeller base plate.

3. Apparatus as set forth in claim 1 in which the impeller blades have a leading flight and a trailing flight and the trailing flight is at a lower angle to the impeller base plate than the leading flight.

4. Apparatus as set forth in claim 1 in which a slot substantially parallel to the impeller base plate extends through the impeller blade near the trailing edge thereof.

5. Apparatus as set forth in claim 1 in which the means for delivering the slurry onto the inner end of the impeller blades comprise a tubular guide secured to the inner ends of the impeller blades, a feed line extending into the guide and terminating at a level spaced from the impeller base plate, the end of the guide adjacent the impeller base plate being spaced therefrom to provide ports for delivery of slurry onto the inner end of the impeller blades.

6. Apparatus as set forth in claim 1 in which the means for separately collecting material discharged from the impeller blades includes a cylindrical wall having an open end terminating adjacent the intersection of the trailing edge and outer end of the impeller blades.

7. Apparatus as set forth in claim 1 in which the impeller blades extend downwardly from the impeller base plate and the means for separately collecting material discharged from the impeller blades includes a cylindrical wall extending downwardly from an upper end adjacent the intersection of the trailing edge and the outer end of the impeller blades.

8. Apparatus as set forth in claim I in which means for delivering the slurry comprise a tubular guide centrally mounted on the upper surface of the impeller base plate, the means for rotating the impeller base plate is secured to the lower surface thereof, the impeller blades extend upwardly from the impeller base plate, and ports in the guide positioned above the impeller base plate allow flow of slurry onto the inner ends of the impeller blades.

9. Apparatus as set forth in claim 1 in which the impeller blades extend downwardly from the impeller base plate, and the means for delivering slurry onto the inner end of the impeller blades comprise an upwardly opening inverted conical guide secured to the inner ends of the impeller blades, the upper end of the guide is spaced from the lower surface of the base plate, and a feed line opens into the conical guide.

10. Apparatus as set forth in claim 1 in which the plurality of radial slots extend through the base plate and tabs at the leading edges of the impeller blades extend through the slots.

11. Apparatus as set forth in claim 1 in which the means for rotating the impeller base plate impart dynamic forces to the slurry substantially greater than gravitational forces.

12. Apparatus as set forth in claim 1 in which the means for rotating the impeller base plate rotate at a rate of at least about 1,700 rpm.

13. Apparatus as set forth in claim 1 for the separation of ferrous abrasive particles from drilling liquid in which the impeller blades are secured to the impeller base plate at an angle with the impeller base plate in the range of 35 to 50.

14. Apparatus as set forth in claim 1 in which the impeller blades have a flat leading flight and are curved along the trailing edge in a direction opposite the direction of rotation of the impeller base plate, and a slot substantially parallel to the impeller base plate extends through the impeller blades over a substantial portion of the juncture of the flat and curved portion of the blades.

15. Apparatus for separating a liquid from solid particles suspended therein comprising a housing having a bottom sloping to an outlet, an open-bottomed chamber extending downwardly through the bottom of the housing, a motor mounted above the housing, a shaft extending from the motor downwardly through the upper end of the housing, an impeller base plate secured to the shaft within the housing, radial impeller blades secured along one edge to the impeller base plate and slating downwardly therefrom with the trailing edge spaced from the impeller base plate, a conical guide open at its upper end secured to the inner end of the impeller blades with the open end of the guide spaced from the lower surface of the base plate, a feed line opening into the conical guide for delivery of the liquid and solid particles into the guide, and the upper end of the chamber is positioned adjacent to the intersection of the trailing edge and outer end of the impeller blades.

'zg g UNITED STATES PATENT omen CERTIFICATE OF CORRECTION Patent No. 3,789,984 mud February 1974 Inventor(s) Eber Gaylord It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corructcd as shown below:

Column 1, line 38, "type" should be ---types- Column 2, line 14, after "ler" insert -blades nearest the center of rotation of the impeller---.

C Column 4, line 49, "alow'! should be --allow.

Column 7, line 1, after "only" insert --to the--.

Signed and sealed this 17th day of- September 1374.

(SEAL) Attest:

MCCOY GIBSQN JR. C. MARSHALL DANN Attestlng Officer Commissioner of Patents 

1. Apparatus for separating solid particles from a slurry comprising an impeller base plate, means for rotating the impeller base plate about a central axis perpendicular thereto, a plurality of uniformly spaced substantially radially extending impeller blades secured to the impeller base plate in an inclined position with the trailing edge of the blades spaced from the impeller base plate and the leading edge at the base plate, means for delivering the slurry onto the end of the impeller blades nearest the axis of rotation and adjacent the impeller base plate, said blades being constructed and arranged so that particles that move more freely on the blades are discharged from the trailing edge of the blades and material that moves less freely is discharged from the outer end of the blades and means for separately collecting material discharged from the trailing edge of the impeller blade and material discharged from the outer end of the blades.
 2. Apparatus as set forth in claim 1 in which the impeller blades are curved along the trailing edge in a direction opposite the direction of rotation of the impeller base plate.
 3. Apparatus as set forth in claim 1 in which the impeller blades have a leading flight and a trailing flight and the trailing flight is at a lower angle to the impeller base plate than the leading flight.
 4. Apparatus as set forth in claim 1 in which a slot substantially parallel to the impeller base plate extends through the impeller blade near the trailing edge thereof.
 5. Apparatus as set forth in claim 1 in which the means for delivering the slurry onto the inner end of the impeller blades comprise a tubular guide secured to the inner ends of the impeller blades, a feed line extending into the guide and terminating at a level spaced from the impeller base plate, the end of the guide adjacent the impeller base plate being spaced therefrom to provide ports for delivery of slurry onto the inner end of the impeller blades.
 6. Apparatus as set forth in claim 1 in which the means for separately collecting material discharged from the impeller blades includes a cylindrical wall having an open end terminating adjacent the intersection of the trailing edge and outer end of the impeller blades.
 7. Apparatus as set forth in claim 1 in which the impeller blades extend downwardly from the impeller base plate and the means for separately collecting material discharged from the impeller blades includes a cylindrical wall extending downwardly from an upper end adjacent the intersection of the trailing edge and the outer end of the impeller blades.
 8. Apparatus as set forth in claim 1 in which means for delivering the slurry comprise a tubular guide centrally mounted on the upper surface of the impeller base plate, the means for rotating the impeller base plate is secured to the lower surface thereof, the impeller blades extend upwardly from the impeller base plate, and ports in the guide positioned above the impeller base plate allow flow of slurry onto the inner ends of the impeller blades.
 9. Apparatus as set forth in claim 1 in which the impeller blades extend downwardly from the impeller base plate, and the means for delivering slurry onto the inner end of the impeller blades comprise an upwardly opening inverted conical guide secured to the inner ends of the impeller blades, the upper end of the guide is spaced from the lower surface of the base plate, and a feed line opens into the conical guide.
 10. Apparatus as set forth in claim 1 in which the plurality of radial slots extend through the base plate and tabs at the leading edges of the impeller blades extend through the slots.
 11. Apparatus as set forth in claim 1 in which the means for rotating the impeller base plate impart dynamic forces to the slurry substantially greater than gravitational forces.
 12. Apparatus as set forth in claim 1 in which the means for rotating the impeller base plate rotate at a rate of at least about 1,700 rpm.
 13. Apparatus as set forth in claim 1 for the separation of ferrous abrasive particles from drilling liquid in which the impeller blades are secured to the impeller base plate at an angle with the impeller base plate in the range of 35* to 50*.
 14. Apparatus as set forth in claim 1 in which the impeller blades have a flat leading flight and are curved along the trailing edge in a direction opposite the direction of rotation of the impeller base plate, and a slot substantially parallel to the impeller base plate extends through the impeller blades over a substantial portion of the juncture of the flat and curved portion of the blades.
 15. Apparatus for separating a liquid from solid particles suspended therein comprising a housing having a bottom sloping to an outlet, an open-bottomed chamber extending downwardly through the bottom of the housing, a motor mounted above the housing, a shaft extending from the motor downwardly through the upper end of the housing, an impeller base plate secured to the shaft within the housing, radial impeller blades secured along one edge to the impeller base plate and slating downwardly therefrom with the trailing edge spaced from the impeller base plate, a conical guide open at its upper end secured to the inner end of the impeller blades with the open end of the guide spaced from the lower surface of the base plate, a feed line opening into the conical guide for delivery of the liquid and solid particles into the guide, and the upper end of the chamber is positioned adjacent to the intersection of the trailing edge and outer end of the impeller blades. 